could you ask his therapists for advice/learning aids?

makes more sense to me, to learn the analogue version first, but i'm not a teacher, or a spectrum therapist, so can only make a general mum comment.

my nearly five year old is learning the "o'clocks", and it is helping her alot.
we began with the calendar, crossing off days, months ago. i didn't see it (at the time), as having anything to do with reading the time, but learning the numbers 1 - 30, has helped her with reading the clock too.

You can buy wall clocks with the numbers, for under $5 at big w etc, (cannot believe how cheap they are), so we're making a special effort to have more clocks like that now (to help her learn the time at mummy's house and daddy's house).

Just had another thought, i have come across good "video tutorials" on youtube, about all sorts of things. Maybe try Youtube. Google "teaching reading the time to children on autism spectrum"? etc.

i guess every kid responds to different things. Does he like books? or being taught orally?
e.g a train book about reading the time would be great, if your child was keen on trains and enjoyed books.